1. Field of the Invention
The present invention relates to a surface protective film and process for producing the same, more particularly a surface protective film which can be easily peeled off from an adhesive silicone rubber composition layer, free of a plasticizer or the like, which may have an adverse effect on adhesion of the adhesive silicone rubber composition layer to a semiconductor chip or portion at which a semiconductor chip, and having no adverse effect on thickness uniformity or surface flatness of the adhesive silicone rubber composition layer, and process for producing the same.
2. Description of the Related Art
A semiconductor device, also referred to as semiconductor equipment, is composed of semiconductor chips encased in a package, and has been used for control and arithmetic circuits for electronic computers, TV sets, DVD's, VTR's, radios, microwave ovens, automobiles, airplanes, chemical plants, and so on.
There are many types of semiconductor devices, and some of the representative ones are illustrated in FIGS. 1 and 2. A semiconductor device is described by referring to FIG. 1. The semiconductor chip 1 (sometimes referred to as silicon wafer), which may be an IC or LIC, is at the center of the device, with its lower side being bonded to the upper side of the adhesive silicone rubber composition layer 2, whose lower side is bonded to the upper side of the semiconductor chip attaching portion 3. This assembly condition is sometimes expressed in this specification that the semiconductor chip 1 is bonded (synonymous with “attached,” which may be interchangeably used) to the semiconductor chip attaching portion 3 via the adhesive silicone rubber composition layer 2.
The adhesive silicone rubber composition layer works to bond the semiconductor chip 1 to the semiconductor chip attaching portion 3, and, at the same time, to relax stress between these components.
The semiconductor chip 1 is connected to the circuit wiring 4 by the bonding wire 5, and the block composed of the semiconductor chip 1, adhesive silicone rubber composition layer 2, semiconductor chip attaching portion 3 and circuit wiring 4 is sealed with a sealant of epoxy-based resin, which protects these components from dust and moisture in the air, impact and so on, and, at the same time, releases heat produced inside into the outside.
A semiconductor chip has been bonded to a semiconductor chip attaching portion by a silicone-based adhesive agent, e.g., of liquid, crosslinkable (synonymous with “curable,” which may be interchangeably used) silicone composition or pasty, crosslinkable silicone composition.
Some of the representative silicone-based adhesive agents are of crosslinkable silicone composition of, e.g., organopolysilioxane having per molecule at least two Si-bonded alkenyl groups or organopolysilioxane having per molecule at least two Si-bonded hydrogen atoms; crosslinkable silicone composition comprising a hydrosilylation catalyst; or crosslinkable silicone composition incorporated with an organopolysiloxane having per molecule at least one Si-bonded alkoxy or alkenyl group, or hydrogen atom, as disclosed by, e.g., Japanese Patent Laid-open Application No.3-157474, where the organopolysiloxane works as an adhesion promoter.
The crosslinkable silicone composition involves a problem that low-viscosity silicone oil oozes out from the composition while being crosslinked to contaminate the composition surroundings. Moreover, the low-viscosity silicone oil is very difficult to completely remove, because it may be an organopolysiloxane of low degree of polymerization contained in an organopolysiloxane as the main ingredient or as an adhesion promoter. This may cause a problem of deteriorated reliability of the semiconductor device, because the assembly comprising a semiconductor chip and semiconductor chip attaching portion bonded to each other by such a curable silicone composition may have deteriorated wire bondability between a bonding pad on the chip and bonding wire or beam lead.
The techniques which have solved these problems include an adhesive silicone rubber composition layer (synonymous with “sheet,” which may be interchangeably used) which can well bond a semiconductor chip and semiconductor chip attaching portion to each other, and hence give a highly reliable semiconductor device, while oozing out of low-viscosity silicone oil is controlled during the crosslinking step; process for efficiently producing the adhesive silicone rubber composition layer for a highly reliable semiconductor device with the adhesive silicone rubber composition layer of particularly high adhesion; and highly reliable semiconductor device with semiconductor chip and semiconductor chip attaching portion bonded to each other by the adhesive silicone rubber composition layer, as disclosed by Japanese Patent Laid-open Application No.11-12546.
The adhesive silicone rubber composition layer may be contaminated with dust or moisture in the air, when its surface is kept exposed, which can deteriorate performance of the semiconductor chip and/or semiconductor chip attaching portion when these components are bonded to the layer. Therefore, the layer should be protected by a protective film against dust and moisture.
The protective films include those of fluorine, polyethylene, polypropylene and polyimide resin, as disclosed by Japanese Patent Laid-open Application No.11-12546. Moreover, Japanese Patent Laid-open Application No.2001-19933 discloses polyester, polyether (conceivably special rubber of polyether), polyether sulfone, epoxy and phenol resin, among others, for the surface protective film.
A fluorine-based film, although good in releasability, involves a problem of reduced adhesive strength of the adhesive silicone rubber composition layer to the semiconductor chip, semiconductor chip attaching portion or the like, after it is peeled off.
A polyethylene or polypropylene resin sheet (synonymous with “film” “layer” or the like, which may be interchangeably used), although low in cost, has a melting point lower than crosslinking temperature of the adhesive silicone rubber composition to cause a problem of deformed surface protective film itself with the result that the adhesive silicone rubber composition layer of constant thickness may be no longer formed.
A polyimide resin sheet, although very high in heat resistance and dimensional stability, involves problems of high cost and slightly low releasability from the adhesive silicone rubber composition layer.
A polyester resin sheet, represented by polyethylene terephthalate (PET) sheet, although favorable in that it is free of fisheye or uneven thickness, involves a problem of poor releasability.
A polyether resin sheet involves problems of insufficient surface flatness, high cost and very poor releasability from the adhesive silicone rubber composition layer.
A polyether sulfone (sometimes referred to as PES) resin sheet has been used as a surface protective film most widely, because of its good releasability from the adhesive silicone rubber composition layer, but involves a problem of causing insufficient punching characteristics of the silicone laminate to increase defective product yield.
An epoxy resin sheet is not used, because of very poor releasability from the adhesive silicone rubber composition layer.
A phenol resin sheet is not used, because of very poor flexibility.
The common releasing film (separator) is difficult to use as the surface protective film, because it is surface-treated with silicone for releasability to deteriorate its releasability from the adhesive silicone rubber composition layer, which is similar to the film surface.
The conventional procedure for bonding the semiconductor chip and semiconductor chip attaching portion to each other with the adhesive silicone rubber composition layer in-between exposes the semiconductor chip to high temperature, because it requires heating the adhesive silicone rubber composition layer at relatively high temperature for extended periods to improve its adhesive strength. This can deteriorate performance of the semiconductor chip. Therefore, the adhesive silicone rubber composition layer tends to be incorporated with a silicone-based adhesion promoter. However, as discussed earlier, use of the silicone-based adhesion promoter may cause problems of deteriorated releasability of the surface protective film from the adhesive silicone rubber composition layer to obstruct the releasing works, and uneven thickness of the adhesive silicone rubber composition layer left by the surface protective film. These problems may cause difficulty in producing semiconductor devices of high quality.